1. Field of the Invention
The present invention relates to a high frequency (HF) circuit in a communication device and more specifically to a technique for bonding a circuit board to a metal chassis or case, a waveguide-microstrip line transition, a branch circuit, and a high frequency circuit incorporating these elements.
2. Description of the Prior Art
Recently, as frequency resources in communications technology are running dry, frequency bands available for building a new communications system have been and are shifting to higher bands. In this situation, the government and the people are jointly promoting a development to milliwave and microwave communication systems domestically and internationally. For example, it has been decided that extremely high frequency bands ranging from some GHz to hundreds GHz are assigned as available frequency bands to various communication systems under development for wireless LAN (local area network) and ITS (Intelligent Transport System).
Since available frequencies are rising as described above, antennas and HF (high frequency) circuits are desired which satisfactorily work in milliwave and microwave bands. However, design and manufacturing techniques that have been believed to be available may not work satisfactorily with an increase in frequency. For this reason, there is a need for novel design and manufacturing techniques.
FIG. 1 is a diagram showing an arrangement of a prior art array antenna assembly 1. In FIG. 1, the antenna assembly 1 comprises an dielectric substrate 10, a circuit pattern 20, a chassis 30 that holds the dielectric substrate 10 and serves as the ground, and a waveguide-microstrip line transition 40. The circuit pattern 20, which constitutes an array antenna, includes a T branch circuit 50. A signal transmitted through a waveguide (not shown) is passed by the transition 40 to a microstrip line of the circuit pattern 20, and further passed by the T branch circuit 50 to the right and the left portions of the array antenna.
FIG. 2 is a schematic diagram showing an arrangement of the transition 40 of FIG. 1. In FIG. 2, the transition 40 comprises a ridge waveguide 42, a ridge 41 formed inside the ridge waveguide 42, and a microstrip line 21 which is formed on the dielectric substrate 10 and which is extending to (or a part of) the circuit pattern 20. As described above, the signal transmitted through the not-shown waveguide is converted into a transmission mode of the microstrip line 21 by the ridge 41 provided inside the waveguide 42 and transmitted to the array antenna 20.
Problems exist in conjunction with working if an antenna with the just-described arrangements are to be implemented for milliwave or microwave. With an increase in frequency, dielectric materials available for the dielectric layer 10 is limited to substances lack of a mechanical strength, e.g., ceramics, quartz, silicon, etc. Further, if an antenna that radiates a beam of two degrees in mesial width in a 76 GHz band is to be fabricated, the dielectric substrate 10 for the antenna will be approximately 100 to 300 xcexcm thick and 15 cm long in one side. Bonding such a thin and wide substrate 10 to the chassis 30 often results in a breakage of the dielectric substrate 10. Also, as the frequency increases, the characteristics of the antenna 20 depends strongly on the grounding state of the dielectric substrate 10. For this reason, a sufficient electrical contact is indispensable for the junction of the dielectric substrate 10 and the circuit pattern 20. However, this is hard to be achieved by conventional techniques.
Since the degree of freedom is very low in designing a waveguide-microstrip line transition, i.e., the design parameters are limited only to the width, the length and the height of the ridge 41, this sometimes causes the width of ridge for a milliwave band to be extremely narrow. Accordingly, the height of the ridge 41 of the transition 40, which is manufactured through machining of a brass material, becomes higher as compared with the ridge 41 width, making the work difficult. The lack of freedom in the design makes transition with a microstrip line having a lower characteristic impedance difficult and cause the problem that too large a difference between the widths of the designed ridge 41 and the microstrip line 21 leads to an unexpected deterioration in the impedance matching characteristics.
As is not limited to a high frequency (HF) antenna, an array antenna 20 as a whole generally exhibits a narrower frequency band characteristic with an increase in the number of array elements. Taking for example an antenna used in a front monitoring radar being put to practical use in 60 GHz, the antenna needs a beam width of about 2 degrees and accordingly a very large size. If a structure incorporating a conventional branch circuit were used as it is for such antenna, the resultant antenna would exhibit a very narrow frequency band characteristic, causing the band width of the antenna to be narrower than that of the radar. This is because conventional branch circuits mainly use stubs for impedance matching. FIG. 3 is a diagram showing an exemplary pattern of a conventional T branch circuit comprising a matching circuit that uses stubs 51 (the T branch circuit is shown as a dark area). Using stubs for impedance matching generally tends to narrow the frequency characteristics of the circuit. Specifically, the larger the distances (D1 and D2) between the matching circuit and circuits (22) that need matching, the narrower the frequency band of the whole circuit. However, if stubs are to close to the antenna (or the circuits that need matching) so as to broaden the frequency band of the antenna, the antenna will fail to provide the desired characteristic. Thus, matching by stubs while providing a desired characteristic to the antenna or the circuits having their impedance matched inevitably narrows the frequency band of the resultant circuit such as an antenna.
The invention is directed to solving these and other problems and disadvantages of the prior art.
It is an object of the invention to provide a technique of bonding a thin and large-area circuit substrate to a metal layer with a sure and uniform contact but no fear of substrate breakage; a waveguide-microstrip transition that has a high degree of freedom in design and easy to work; and a branch circuit that permits the frequency band of circuit to be wide.
It is another object of the invention to provide a high frequency circuit and an antenna that incorporate an circuit substrate implemented by such a bonding technique, such a waveguide-microstrip transition and such a branch circuit, and to provide a communication system using such a high frequency circuit and such an antenna.
According to an aspect of the invention, a method of bonding a circuit board with a metal plate is provided. The method includes the steps of working the metal plate so as to have a shape that permits a fluid to form a bath in an area including a part where the circuit board is to be bonded; heating the worked metal plate to such a temperature as melt a conductive bonding material; forming a bath of the conductive bonding material in the area of the metal plate; floating the circuit board on the bath; and absorbing excessive portion of the conductive material without applying a force to the dielectric substrate.
A circuit assembly according to just-described aspect of the invention is provided with a thin and large-area dielectric substrate with an improved grounding condition. A bonding agent with a low melting point, a low melting point solder, etc. may be used as conductive material.
According to another aspect of the invention, a branch circuit for branching a first path into at least two second paths in a high frequency circuit is provided. The impedance matching between the first path and each of the branch paths is achieved by mainly using impedance transformers but by using fewest possible stub(s) in the branch circuit. The first path, the second paths, the impedance transformers, and the fewest possible stub(s) are arranged in symmetry with respect to a plane of symmetry that runs through the first path.
In one embodiment, the impedance transformers are step impedance transformers.
According to another aspect of the invention, a waveguide-microstrip line transition that is easy to work and low in transition loss is provided. The transition comprises a fanwise tube having a first opening coupled with a waveguide and a second opening larger in size then the first opening, a first and a second wider wall of the tube spreading from the first opening toward the second opening; an end portion of a microstrip line formed on a dielectric substrate arranged near the first wider wall, the end portion being situated a little inside the second opening and on a plane of symmetry for the first and the second wider walls; and a ridge formed on the second wider wall, the ridge protruding gradually from a first opening side toward a second opening side to become short-circuited, at the end thereof, with the end portion of the microstrip line, wherein dimensions of the fanwise tube and a shape of the first and the second wider walls are determined so as to fit the width of the microstrip line to the end portion of the microstrip line.
In one embodiment, at least a part of each longitudinal side of a fanning-out portion of the wider walls is linear. However, at least a part of each longitudinal side may accords substantially with an exponential function or a trigonometric function.